SEOUL (Yonhap) -- South Korean President Lee Myung-bak and U.S. President Barack Obama strongly urged North Korea to immediatly halt its provocative actions, saying threats and belligerent behavior will result in serious consequences. During their June 16 summit in Washington, the leaders affirmed that peaceful coexistence is still an option for the communist North, but that it can only be obtained through peaceful negotiations.

"I want to be clear that there is another path available for North Korea. A path that will lead to peace and prosperity," the U.S. president said in a joint press conference with Lee shortly after their summit talks at the White House. "That destination can only be reached through peaceful negotiation and denuclearization," Obama said.

The South Korean president said North Korea must understand that "they will not be able to gain compensation by creating crises." "President Obama and I urge the North Koreans to fully give up their nuclear ambitions and become a responsible member of the international community," he added. The South Korean head of state arrived in Washington June 15 on a three-day official visit.

The meeting between Lee and Obama came as Pyongyang is threatening to reinforce its nuclear arsenal and continue with its provocative nuclear and ballistic missile tests despite a U.N. Security Council resolution that condemned its May 25 nuclear test detonation and prohibited any future tests by the communist state.

Obama said Seoul, Washington and the rest of the world will begin "serious enforcement" of sanctions already placed on the North by U.N. Security Council resolutions if Pyongyang continues to provoke.

Lee called for close cooperation between Seoul and Washington in dealing with Pyongyang, saying a firm alliance between the two will make North Korea think twice before taking any steps it might regret.

President Lee also hinted at a possible shutdown of a lucrative industrial park in North Korea. Lee's warning came shortly after Pyongyang demanded in recent dialogue with Seoul that South Korean businesses at the joint industrial complex in Kaesong raise the salaries of their North Korean workers by nearly four times and pay hundreds of millions of dollars in fees.

"We urge North Korea not to make any unacceptable demands because we really do not know what will happen if they keep on this path," Lee told the press conference, adding that more than 40,000 North Koreans currently working for South Korean firms will also lose jobs if the industrial park closes.

The dramatic change of position on Kaesong by Lee, who earlier said the reconciliatory project must be kept alive despite the nuclear standoff, apparently comes as he and Obama agreed to break what they called a pattern of their making concessions following North Korean threats and provocations.

"I want to emphasize something President Lee said, that there has been a pattern in the past where North Korea behaves in a belligerent fashion and, if it waits long enough, it is rewarded," Obama told the joint press conference. "I think that is the pattern they have come to expect. The message we are sending them is that we are going to break that pattern," he added.

The Lee-Obama summit came amid North Korea's threat to reinforce its nuclear arsenal and start enriching uranium -- a move that prompted the ongoing nuclear standoff in the first place -- in a clear defiance of a U.N. Security Council resolution that condemned its May 25 nuclear test. The socialist state has also said it will not come to the negotiating table unless Washington first recognizes it as a nuclear state.

Obama made it clear his country will not make any concessions. "North Korea also has a track record of proliferation that makes it unacceptable for them to be a nuclear power," he told the press conference.

Shortly after the North's nuclear detonation test last month, the second of its kind since 2006, the U.S. said it was considering putting North Korea back on its list of terrorism-sponsoring states. North Korea was taken off the list for first time last year under a denuclearization-for-aid deal.

"In order to take the path of peace and prosperity, North Korea has to make a decision and understand that prestige and security and prosperity are not going to come through the path of threatening neighbors and engaging in violations of international law," he said.

The U.S. president added that the world will begin "serious enforcement" of sanctions already placed on the North by U.N. Security Council resolutions if Pyongyang continues to be provocative and belligerent.

Following their talks, Lee and Obama said they have agreed to significantly boost their countries' relationship, and the South Korean president noted that a firm alliance between the two will make North Korea think twice before taking steps it might regret.

In a joint statement issued at the end of the summit, the leaders said the countries will turn their alliance, formed under a mutual defense treaty signed in 1953, into a "comprehensive, strategic relationship" built not only on bilateral or security issues, but also on social, economic and political issues at regional as well as global levels.

"Through our Alliance we aim to build a better future for all people on the Korean Peninsula, establishing a durable peace on the Peninsula and leading to peaceful reunification on the principles of free democracy and a market economy," said the statement, titled "Joint Vision for the Alliance" of South Korea and the United States.

Experts explained that the leaders of South Korea and the United States tried turning the table on North Korea, telling the communist nation to either choose peaceful coexistence or face further isolation and other serious consequences.

The high-toned words came amid concerns over a possible third nuclear detonation and long-range missile tests by the North. "We are more than willing to engage in negotiations to get North Korea on a path of peaceful coexistence with its neighbors, and we want to encourage their prosperity," Obama said in the joint press conference. "But belligerent, provocative behavior that threatens neighbors will be met with significant, serious enforcement of sanctions that are in place," he said.

Shortly after Pyongyang conducted its second nuclear test on May 25, the South Korean president said the North should not be engaged just for the sake of negotiations, which would be a reward for bad behavior.

The leaders' statement is likely to be met by further provocations from North Korea, however, which last week said it will weaponize its plutonium and start enriching uranium, a technology that can be used as an alternative to making atomic weapons. "North Koreans must understand that they will not be able to gain compensation by provoking a crisis. This has been a pattern in the past, but this will no longer be," President Lee said.

Lee and Obama warned the North against a provocative reaction, reaffirming the U.S. provision of extended deterrence, including a nuclear umbrella, to South Korea. "We will maintain a robust defense posture, backed by allied capabilities which support both nations' security interests. The continuing commitment of extended deterrence, including the U.S. nuclear umbrella, reinforces this assurance," the leaders said in a joint statement, named "Joint Vision for the Alliance" of South Korea and the United States.

Washington has provided nuclear protection to Seoul since the end of the 1950-53 Korean War and frequently reaffirmed its commitment to the joint defense of South Korea. But this was the first time that the pledge was affirmed in a written document signed by a U.S. president, South Korean officials emphasized.

The conservative South Korean leader has not provided food or fertilizer aid to North Korea since taking office early last year, calling for progress first in talks on denuclearizing the North, although his predecessors had provided about 500,000 tons of food and roughly as much fertilizer annually to the impoverished neighbor.

Despite the gloomy outlook for the nuclear talks, the Lee-Obama summit has helped the two nations consolidate their bilateral alliance as well as cooperation in staving off a possible attack from the North, which is widely believed to be equipped with a nuclear arsenal as well as long-range missiles.